With the progress that has been made in recent digital signal processing techniques and semiconductor technology, consumer digital video standards have been proposed for digitally recording moving picture signals according to a standard television scheme such as NTSC or PAL, and digital video cameras obtained by integrating a digital video recording/playback apparatus and an image sensing apparatus have become available commercially as an application of these standards. Such a digital video camera exploits the digital recording capability and sometimes is equipped with a still-picture recording function. Further, there are digital video cameras equipped with a digital interface for connection to a computer or the like and having a function for loading a captured image into the computer. Furthermore, there are digital video cameras equipped with a plurality of writing/reading units conforming to plural kinds of recording media that can be selected among in accordance with the purpose for which an image is used.
In a case where an image that has been recorded by such a digital video camera is reproduced in a television by connecting the digital video camera to the television, there may be no problem whatsoever with regard to an image composed of 720×480 pixels, for example, which is defined by digital video standards. However, if the image is transferred to another medium via a digital interface, there are instances where a larger number of pixels is required for better image quality.
Further, an increase in the number of pixels possessed by image sensing devices has been accompanied by the need to drive such image sensing devices at higher frequencies in order to read out the information represented by all pixels of the device. This invites a decline in S/N ratio and an increase in power consumption.
One example of a method of raising the data rate of sensed-image information while suppressing the driving frequency of an image sensing device is a method of splitting the sensed-image screen into a plurality of areas, providing an independent charge transfer section, amplifier and output terminal for each area and reading sensed-image signals output of these areas in parallel. An example of a prior-art image sensing apparatus using such an image sensing device is illustrated in FIG. 8. As shown in FIG. 8, the screen of an image sensing device 800 is split into two, namely left and right, areas 801, 802 each having a photoelectric converter and vertical transfer section. The apparatus has horizontal transfer sections 803, 804, amplifiers 805 and 806 and output terminals 807, 808. Using an image sensing device having such a structure is advantageous in that sensed-image information is obtained at a data rate that is twice the driving frequency of the image sensing device.
The image of a subject that has been formed on the image sensing device 800 by an image forming optical system (not shown) is converted to an electric signal by the image sensing device 800, and electric signals are output from the output terminals 807, 808 in accordance with driving pulses supplied from a drive timing generating circuit, not shown.
The two image signals obtained from the image sensing device 800 are subjected to analog signal processing and then to an analog-to-digital conversion by analog front ends 809, 810, the digital signals are amplified to a prescribed level by amplifiers 813, 814, and then the left and right images are combined as a single image by a screen combining circuit 817. The output signal of the screen combining circuit 817 is subjected to gamma correction processing, contour correction processing and color correction processing, etc., by a camera signal processing circuit 818, and the result of processing is output from an output terminal 819 as a luminance signal and color difference signals (for example, see the specification of Japanese Patent Application Laid-Open No. 05-022667). The image sensing apparatus further includes a dB/linear conversion circuit 812 for converting a logarithmic value to a linear signal and a gain control circuit 815 for controlling the gain value during gain adjustment.
In this example of the prior art, however, the characteristics of the amplifiers and peripheral circuits provided for the respective areas are not uniform. As a result, a problem which arises is that if an image is generated by combining two areas, a decline in image quality occurs such as the occurrence of a boundary line ascribable to a difference in levels between the areas.
In order to deal with this problem, a method of correcting the level difference between the areas has been proposed in the specifications of Japanese Patent Application Laid-Open Nos. 2002-125149 and 2002-142158, by way of example. In these examples of the prior art, it is described that the gains and offsets of the left and right channels are adjusted to correct the level difference. The gain and offset values used are found in accordance with the difference between the image signals on the two channels obtained by projecting a prescribed amount of light from an LED upon the image sensing apparatus when a calibration command is received. However, regardless of the fact that the gain balance of the left and right channels for which the level difference can be adjusted varies in accordance with the size of the gain applied to the image signal, in other words, in accordance with the brightness of the image, the gain is acquired based upon an image signal of an image of substantially constant brightness at all times obtained by the projection light from the LED. Consequently, depending upon the brightness of the subject, there are instances where the level difference between the left and right channels cannot be corrected fully even if the correction is applied using the acquired gain.